1. Field of Invention
The invention relates to a calibration device for mass flow meters and a method for calibrating a mass flow meter using such a calibration device, and more particularly, to a calibration device having at a test piece measuring section, a device for creating a flow of a medium through the test piece measuring section and a temperature-measuring device in the test piece measuring section for detecting the temperature of the medium and a method for calibrating a mass flow meter using such a calibration device.
2. Description of Related Art
Calibration devices of the above-mentioned type have been known for a long time from the related art. Calibration serves the purpose of detecting the deviation of the measurement of the mass flow meter test piece from a standard value provided by the calibration device in order to calibrate the mass flow meter test piece using the deviation. Such calibration devices are also used in the calibration of mass flow meters, wherein, the conformity with certain accuracy requirements is approved by certified institutes, such as, for example, the German Physikalisch-Technische Bundesanstalt. The requirement of calibration and repeated calibration of a mass flow meter results in part from the field of application of mass flow meters, for example, the transportation of oil for custody transfer in which only calibrated mass flow meters may be used.
Additionally, a high accuracy is always desired from a technical perspective. In particular, regarding high-grade fluid or gaseous goods, such as crude oil and natural gas, there is a large interest, from the distributors point of view, that the amount to be delivered and only that amount is actually delivered, and from the purchasers point of view, that the requested amount, especially at least the requested amount, is received. Measurement tolerances always impact one of the parties in trade and they usually impact the distributor.
The standard value with which the measurement of the mass flow meter test piece is compared can exist in the form of a standard measuring device, which is also placed in the test piece measuring section. In this manner the standard measuring device is subjected to the same flow as the mass flow meter test piece to be calibrated arranged at a distance away in the test piece measuring section. This is valid, in particular in gaseous media, insofar as other variables influencing the mass or volume flow within the medium are stationary, or at least are stationary for as long as the standard measuring device and the mass flow meter test piece have carried out their measurement under the same conditions. Normally, calibration devices also include pressure measuring devices, since, in particular in gaseous media, the pressure has a large influence on the density of the medium. Thus, pressure represents an important variable for such mass flow meters that are based on the measurement of the flow speed of the medium, as is the case in, for example, with the use of ultrasonic mass flow meters, as opposed to measuring devices that allow a direct conclusion about the mass flow due to their measuring principle, for example, in Coriolis mass flow meters.
Often, a standard value is also implemented as a volumetric standard value in which a geometrically measured reference volume, for example, in the form of a plunger system, is pushed in the volume of the test piece measuring section in a certain time such that the flow can be adjusted very accurately using the test piece measuring section.
The temperature-measuring device in the calibration device, mentioned above, is necessary or advantageous for an accurate calibration for different reasons. Firstly, the exact detection of the temperature of the medium is of interest for calibrating mass flow meter test pieces at different operating temperatures. Secondly, the temperature of the medium also substantially influences the calibration device. For example, geometric measurements of the tube system of the calibration device are dependent on the temperature of the medium, in particular thermal expansion or contraction.
It is known from the related art to detect the temperature of the medium with high accuracy using invasive temperature probes, i.e., temperature sensors that extend into the volume flowing through the test piece measuring section. Generally, temperature probes are used that are based on changes in electrical impedance, for example, platinum temperature sensors that are arranged in the tip of a measuring tube extending into the flow of the medium. In this instance, temperature measurement occurs with high accuracy. However, the invasive temperature probe has the disadvantage that the flow at the measurement point and downstream from the measurement point have substantial disturbances such that the flow in the calibration device cannot be produced as steadily, in total, as is necessary for a highly accurate measurement. A further disadvantage is that an invasive temperature-measuring device only provides selective temperature information, which does not allow the measurement of temperatures changing along a flow profile, i.e., the identification of a temperature profile. This can be avoided by measuring the temperature with temperature probes at different points in the flow profile or in a flow cross-section. However, this is very disadvantageous because the number of disturbances induced in the flow to be measured is then increased.